Modulation system



-7, 9 c. A. CULVER 1,896,742

I MODULATION SYSTEM Filed Jan. 23, 1952 2 Sheets-Sheet l INVENTOR Eharlns A. Culver BY ATTORNEY Feb. 7, 1933. c, A. CULVER MODULATION SYSTEM Filed Jan. 23, 1932 2 Sheets-Sheet 2 H Y RE M O R m m E T W A%A Patented Feb. 7, 1933 CHARLES .A. C'ULVER, NORTHFIELD, MINNESOTA MODULATION SYSTEM Application filed January 23, 1932. Serial No. 588,277.

' My invention pertains in general to high frequency communication systems and specifically relates to means for modulating'high frequency energy. 1 7

One of the objects of my invention consists in providing a system for modulating high -frequency'energy which is eliicient in 1ts operationand which materially reduces distortion and other disturbances commonly associated with high frequency modulation systems.

Another object consists in producing a modulation system in which the core of the modulating transformer is not subject to any magnetic flux except that due to modulating currents in the windings of the transformer. A further object comprises providing a modulation system having a balanced audio frequency amplification system and a balanced high frequency amplification system coupled by a transformer having windings connected at their midpoints to a direct current supply for both amplifier systems.

These and other desirable objects will be apparent from the following taken in conjunction with the accompanying drawings in which like reference numerals designate corresponding parts throughout and in which; r

' Fig. l is a diagrammatic representation of one embodiment of the modulation system of my invention; and

Fig. 2 is an alternative arrangement of the circuit shown in Fig. 1.

In United States Patent #1,? 90,57 6, issued January 27, 1931, I have disclosed a modulating system including a balanced audio frequency amplifier system for controlling a high frequency oscillator circuit through a transformer. The circuit arrangements disclosedin this patent are such that flux in the core of the transformer due to direct currents in the primary winding of the transformer are avoided thereby reducing distortion due to high flux density. My present invention, in addition, avoids any flux due to direct currents in the secondary winding of the modulation transformer whereby the transformer will operate in accordance with laws holding for alternating current transformers. Other the cathodes and through a source of direct current 22 to the .23 toa microphone 2 1. The circuit 2123- advantages to be derived from my modulatlon system reside in the various circuit arrangements, particularly the use of a pushpull radio frequency amplification organization.

Referring to the drawings in detail, and particularly to Fig. 1, there is provided a source of high frequency energy 1, connected with a circuit including inductance 2 and capacitance 3. The source of high frequency 63 energy 1, is indicative of any means for producing highfrequency oscillations suitable for radio or carrier current transmission purposes. The inductance 2 is electromagnetically coupled to an inductance 4 connected 65 to the control electrodes of space discharge tubes 5 and 6.

Space discharge tubes 5 and 6 are of the screen grid type and are arranged in a pushpull circuit. The screen electrodes of the tubes 5 and 6 are connected through sources of direct current 7 and 8, respectivel to ground. The source of direct current 9 is provided between the ground and a midpoint of the inductance 4 and 75 serves-as a source of biasing potential for the control electrodes of tubes 5 and 6. The anode electrodes of the tubes 5 and 6 are connected, respectively, through high frequency choke coils 10 and 11 to opposite sides of the secondary winding 12 of an audio frequency transformer having a primary winding 13. Separate sources of direct current 141 and 15 arerespectively connected through variable resistances 16 and 17 to midpoints of the windings 12 and 13, respectively.

The winding 13 is connected to the anodes of space discharge tubes 18 and 19, respectively, the control electrodes of which are connected to opposite ends of a secondary winding 20 of a transformer having a primary winding 21. A connection is provided from the cathodes of the tubes 18 and 19 midpoint of the winding 20' and serves as a source of biasing potential for the control electrodes of tubes 18 and 19. The winding 21 is connected through a source of current 24 is indicative of any source of modulation energy.

The circuits between the anodes of space discharge tubes 5 and 6 and the radio frequency choke coils 10 and 11 are extended through capacitances and 26 to incluc e an inductance 27 electromagnetically coupled to an inducta. ce 28. The chokes 10 and 11 prevent the high frequency currents from flowing to the winding 12 while condensers and 26 prevent the direct current from flowing through inductance 2T. Connectioi'is are also provided through capacitances 29 and to ground, as shown. The inductance 28 is connected in circuit with a load 31 which is indicative of any work circuit for the ultilization of modulated high frequency energy produced according to my system.

In the operation of my system for producing modulated high frequency energy, the

microphone 24: produces a variable current in the winding 21 whereby electromotive forces are induced in the winding 20 for controlling the balanced amplifier tubes 18 and 19. The tubes 18 and 19 serve as variable resistances for controlling the current flowin the primary winding 13 as supplied from the source 15 through control resistance 17..

The source of high frequency energy 1 acts through inductance 2 to induce high frequency electromotive forces in the inductance 4,- for controlling the balanced h h frequency amplifier tubes 5 and 6. The anode voltage for the tubes 5 and 6 is supplied from source 11 connected through resistance 16 to the midpoint of winding 12. The magnitude of the direct potentials impressed on the electrodes of the tubes 5 and 6 are such that the operation of these tubes takes place on the middle point of their dynamic characteristics during periods of no modulation. The variable currents in the winding 13 under control of tubes 18 and 19 will induce alternating electromotive forces in the winding 12 which will be algebraically added to the fixed values of direct potential applied to the anodes of the tubes 5 and 6. The resultant potential of these anodes may therefore vary between desired values, zero and twice the quiescent value being the lower and upper limits. The modulated high frequency energy acts through inductance 27 to induce modulated high frequency electromotive forces in the inductance 28 for utilization in the load 31.

Fig. 2 is an alternative form of the circuit arrangement of Fig. 1 in which the in ductance 1 is connected with amplifier tubes and 36 of the three-element or triode type. no direct current supply to the windings 12 and 13 is in this instance effected from a single source of supply 37 having variable re sistances 16 and 17 respectively connected with midpoints of windings 12 and 18. The control electrodes of the tubes 35 and 36 are coupled to the inductance 27 through capacitances 39 and 40 for neutralizing purposes. Other circuit components are identical with those shown in Fig. 1.

It will now be apparent that my invention provides an efiicient system for effecting the modulation of high frequency energy. The principal advantages of my system reside in the use of a balanced or push-pull high frequency amplifier and a balanced audio frequency amplifier which are coupled by a transformer having windings with midpoint connections to direct current supply. Such a system eliminates direct current flux in the modulation transformer and considerably reduces modulation distortion thereby accomplishing increased fidelity and efficiency in modulation.

Although I have shown a preferred embodiment of my modulation system it will of course be evident that changes can be made therein without departing from the intended scope of my invention. I do not therefore desire to limit myself to the foregoing except insofar as may be pointed out in the appended claim.

What I claim as new and original and desire to secure by Letters Patent of the United States is:

A modulation system comprising, a source of high frequency energy, a high frequency amplification system including space discharge tubes having input and output circuits respectively interconnected in balanced formation, means for coupling said input circuits to said source of high frequency energy, a source of modulation energy, a modulation energy amplification system including space discharge tubes having input and output circuits respectively interconnected in balanced formation, means for coupling said source of modulation energy to said input circuits of said last mentioned amplification system, a transformer having windings one of which is connected with the output circuits of the space discharge tubes of said modulation energy amplification system and the other of which is connected with the output circuits of the space discharge tubes of said high frequency amplification system, resistor controlled means for supplying direct current to the midpoints of said windings, a utilization circuit capacatively connected to the anodes and cathodes of each of the space discharge tubes of said high frequency amplification system, and choke coils in the output circuits of the space discharge tubes of said high frequency amplification system for preventing the passage of high frequency energy to said transformer.

CHARLES A. GULVER. 

